1. Field of the Invention
The present invention relates to a lithographic apparatus, a patterning assembly and a contamination estimation method.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
The lithographic apparatus includes a patterning device configured to impart a radiation beam with the pattern in its cross-section to form the patterned radiation beam. The patterning device is supported by a pattern support making the patterning device exchangeable, thus making it possible to obtain a variety of patterns by exchange of the patterning device. The lithographic apparatus commonly operates in a clean environment, i.e. an environment in which contamination, dust, or any other type of pollution has been reduced to a minimum. Nevertheless, some degree of contamination on the patterning device or on the pattern support is observed. Such contamination may include any type of particles, and may be in an order of magnitude of a nanometer range to micrometer range in thickness thereof. A consequence of this contamination is a deformation of a shape of the patterning device leading to positional inaccuracies (X/Y) and/or focus loss (Z) during exposure. A further adverse effect of such contamination is that it may result in slippage of the patterning device with respect to the pattern support when moving the pattern support. The lithographic apparatus frequently make use of a patterning device which performs a scanning movement, thus the patterning device and pattern support being subjected to acceleration and deceleration forces. As these forces act on the pattern support (actuators to move the patterning device and pattern support act on the pattern support), slippage of the patterning device with respect to the pattern support may occur. In addition, the patterning may be held onto the pattern support using a vacuum suction device. A contamination may thus result in a reduction of an effective force with which the patterning device is held by the pattern support.